Charging unit for a contactless transfer of electric power as well as a power absorbing device and a charging system

ABSTRACT

The present invention relates to a charging unit used for a contactless transfer of electric power to a mobile unit, a complementary power absorbing device and a charging system. The charging unit transfers by means of an alternating magnetic field electric power from at least one primary winding to at least one secondary winding arranged in said mobile unit, and comprises a substantially U-shaped ferrite core which comprises a base area and two leg areas, each provided with a pole face, and which has the primary winding arranged thereon. In order to provide a charging unit for a contactless transfer of electric power to a mobile unit, an associated power absorbing device and a charging system, which permit an improved magnetic coupling between the primary side and the secondary side and consequently an improved power transfer, the primary winding is arranged on the base area of the U-shaped ferrite core. According to an advantageous further development, the base area of the U-shaped ferrite core can be longer than the leg areas so that the ferrite core has a substantially elongate structural design.

[0001] The present invention relates to a charging unit used for acontactiess transfer of electric power to a mobile unit, a complementarypower absorbing device and a charging system according to the genericclauses of claims 1, 4 and 7.

[0002] A contactless transfer of power according to the principle ofinductive coupling is used e.g. for charging accumulators in a mobileelectric unit, such as a mobile radio communication device, a cordlesstelephone, and it is also used in the field of automotive engineering.For such contactless power transfer, power is inductively transferred bymeans of an alternating magnetic field from a charger to the mobileunit, as known e.g. from Offenlegungsschrift DE 197 41 279 A1. Theprimary winding and also the secondary winding are arranged on the twolateral legs of essentially U-shaped ferrite cores. Other known shapesare rod-shaped or E-shaped ferrite cores. In the case of conventionalcharging systems used for contactless power transfer, the ferrite coresof the primary side and of the secondary side have identical dimensions.

[0003] The known embodiments of the ferrite cores entail the problemthat the intrinsic magnetic coupling caused by stray inductances betweenthe pair of poles of each core is excessively high, which has the effectthat the coupling between the primary side and the secondary side isinsufficient. If there is e.g. only a coupling of 50%, so that only halfthe power will be transferred, very high magnetic field strengths willadditionally occur in the primary core when the cores have identicaldimensions. This leads to high core power losses and to a saturation ofthe core. Identically sized pole faces additionally entail the problemthat the magnetic coupling will be reduced when the two pole faces aredisplaced relative to one another.

[0004] It is therefore the object of the present invention to provide achanging unit for a contactless transfer of electric power to a mobileunit, a complementary power absorbing device as well as a chargingsystem, which permit an improved magnetic coupling between the primaryside and the secondary side and, consequently, an improved transfer ofpower.

[0005] This object is achieved by a charging unit, a power absorbingdevice as well as a charging system, which have the features of claims1, 4 and 7. Further developments of the present invention are thesubject matters of a plurality of subclaims.

[0006] By arranging the secondary and/or the primary winding on the basearea of the U-shaped ferrite core, the stray inductances can be kept aslow as possible. The improved coupling leads to a higher overallefficiency; this permits either an increase in the power that can betransferred relative to the core volume or the use of smallertransformers for transferring the same power. Such a transformer savescosts and is advantageous as far as the demands for a far-reachingminiaturization of the components are concerned.

[0007] According to a preferred embodiment, the base area of theU-shaped ferrite core of the secondary side and/or of the primary sideis longer than the leg areas so that the ferrite core has asubstantially elongate structural design. This structural design of thecore leads to an improved magnetic coupling. The length ratio of thebase area to the leg area may here assume values between 2:1 and 4:1.

[0008] When a charging system used for a contactless transfer ofelectric power to a mobile unit is so conceived that the primary-sideferrite core has larger dimensions than the secondary-side ferrite core,this will be advantageous insofar as the core power losses can be keptlow and a saturation of the core can be avoided even if aneconomy-priced standard ferrite material is used.

[0009] When especially the pole faces of the primary-side ferrite corehave larger dimensions than the pole faces of the secondary-side ferritecore, a reduction of the magnetic coupling in the case of a geometricdisplacement of the two pole faces relative to one another can beavoided.

[0010] Making reference to the preferred embodiments shown in thedrawings enclosed, the present invention will be explained in detail inthe following. Similar or corresponding details are provided withidentical reference numerals in the figures, in which

[0011]FIG. 1 shows a side view of a U-shaped ferrite core;

[0012]FIG. 2 shows an additional side view of the U-shaped ferrite core;

[0013]FIG. 3 shows the arrangement of the windings.

[0014]FIGS. 1 and 2 show side views of a ferrite core 100 which can, inprinciple, be used on the primary as well as on the secondary side inaccordance with the present invention. The ferrite core 100 issubstantially U-shaped, since this shape causes the best possiblecoupling between the primary side and secondary side. The ferrite core100 comprises a base area 102 and two leg areas 104, 106, each of saidleg areas 104, 106 being provided with a pole face 108. According to thepresent invention, the winding (only shown in FIG. 3) is arranged on thebase area 102 of the U-shaped ferrite core 100. In order to keep themagnetic coupling between the two pairs of poles of a core 100 to a lowvalue, it is important to keep the ferrite core 100 flat in comparisonto its length. As can be seen from FIG. 1, the length 110 of the basearea is two to three times longer than the outer length 112 of the legareas 104, 106. The ratio of the inner length 114 of the base area 102to the inner length 116 of the leg areas 104, 106 is approximately equalto 4:1, said inner length 116 being shown in FIG. 2. The depth 118 ofthe ferrite core 100 is slightly larger than the outer length 112 of theleg areas. In this way, a particularly compact structural design of thesecondary side as well as of the primary side of the transformer can beachieved. When the ferrite core is used in a mobile phone, the length110 of the base area is e.g. 20 to 30 mm.

[0015] Although the ferrite cores shown in FIGS. 1 and 2 arefundamentally identical in shape for the primary side and for thesecondary side, the primary-side ferrite core 100 has largercross-sectional dimensions 110, 112, 114, 116 and 118 than thesecondary-side ferrite core. In particular the pole faces of theprimary-side ferrite core are larger than the pole faces 108 of thesecondary-side ferrite core. This has the effect that, even if the polefaces are displaced relative to one another, which may happen when amobile unit is placed relative to a charging unit, a satisfactorymagnetic coupling can still be guaranteed. A possible area ratio of theprimary-side pole faces to the secondary-side pole faces is e.g. 1.7:1.However, depending on the respective case of use, also lower or highervalues will provide satisfactory results.

[0016]FIG. 3 shows schematically the arrangement of the windings 120,122 on the base areas 102 of the U-shaped ferrite cores. The primaryside is designated by reference numeral 124 and the secondary side byreference numeral 126. For the sake of simplicity, the different sizesof the primary-side and of the secondary-side ferrite core are not shownin this figure.

1. A charging unit used for a contactless transfer of electric power toa mobile unit, the charging unit transferring by means of an alternatingmagnetic field electric power from at least one primary winding (120) toat least one secondary winding (122) arranged in said mobile unit, andcomprising a substantially U-shaped ferrite core (100) which comprises abase area (102) and two leg areas (104, 106), each provided with a poleface (108), and which has the primary winding (120) arranged thereon,characterized in that the primary winding (120) is arranged on the basearea (102) of the U-shaped ferrite core (100).
 2. A charging unitaccording to claim 1, characterized in that the base area (102) of theU-shaped ferrite core (100) is longer than the leg areas (104, 106) sothat the ferrite core (100) has a substantially elongate structuraldesign.
 3. A charging unit according to claim 2, characterized in thatthe length ratio of the base area (102) to the leg area (104, 106)assumes values between 2:1 and 4:1.
 4. A power absorbing device in amobile unit used for a contactless absorption of electric power, saiddevice absorbing by means of an alternating magnetic field electricpower from at least one primary winding (120) arranged in the chargingunit through at least one secondary winding (122) arranged in saidmobile unit, and comprising a substantially U-shaped ferrite core (101)which comprises a base area (102) and two leg areas (104, 106), eachprovided with a pole face (108), and which has the secondary winding(122) arranged thereon, characterized in that the secondary winding(122) is arranged on the base area (102) of the U-shaped ferrite core(101).
 5. A device according to claim 4, characterized in that the basearea (102) of the U-shaped ferrite core (101) is longer than the legareas (104, 106) so that the ferrite core (101) has a substantiallyelongate structural design.
 6. A device according to claim 5,characterized in that the length ratio of the base area (102) to the legarea (104, 106) assumes values between 2:1 and 4:1.
 7. A charging systemused for a contactless transfer of electric power from a charging unitto a mobile unit, said the charging unit transferring by means of analternating magnetic field electric power from at least one primarywinding (120) arranged in said charging unit to at least one secondarywinding (122) arranged in said mobile unit, and said charging unit aswell as said mobile unit comprising a substantially U-shaped ferritecore (100, 101) which comprises a base area (102) and two leg areas(104, 106), each provided with a pole face (108), characterized in thatthe primary-side ferrite core (100) has larger cross-sectionaldimensions than the secondary-side ferrite core (101).
 8. A systemaccording to claim 7, characterized in that the pole faces (108) of theprimary-side ferrite core (100) have larger dimensions than the polefaces (108) of the secondary-side ferrite core (101).
 9. A systemaccording to claim 8, characterized in that the ratio of the areas ofthe primary-side pole faces to the areas of the secondary-side polefaces assumes values between 1.5:1 and 2.5:1.
 10. A system according toone of the claims 7 to 9, characterized in that the base areas (102) ofthe U-shaped ferrite cores (100, 101) are longer than the leg areas(104, 106) thereof so that both ferrite cores (100, 101) have asubstantially elongate and flat structural design.
 11. A systemaccording to one of the claims 7 to 10, characterized in that thesecondary winding (122) and/or the primary winding (120) are provided onthe base areas (102) of the U-shaped ferrite cores (100, 101).